The Critical Role of Bellows in Control Valves: Selection, Application, and Best Practices

1. Core Function and Primary Types

• Containing Hazardous Media: Preventing the leakage of toxic, volatile, or expensive process fluids (e.g., chlorine, silicon powder) into the environment.
• Excluding Contaminants: Shielding the valve internals from external pollutants like dust, which could cause contamination, abrasion, or valve seizure.
• Protecting Valve Components: Shielding the valve stem from direct (erosion) and corrosion by the medium.

2. Material Selection: Plastic vs. Metal Bellows

• Advantages: Cost-effective, excellent corrosion resistance to acids like hydrochloric and formic acid, and simple manufacturing. They are widely used across industries.
• Limitations: Application is constrained by temperature (usually ≤180°C), pressure (generally ≤12 bar/174 psi), and chemical compatibility. Certain media chemically similar to PTFE can cause swelling ("like dissolves like"). Service life is typically around 5 years.

• Formed Bellows: Made by mechanical hydroforming. They offer a good cost-performance balance and are widely available from numerous manufacturers.
• Welded Bellows: Constructed by welding individual diaphragms. They provide superior precision, longer life (often 10+ years), and better performance but at a higher cost, especially in special alloys. They are essential for ultra-hazardous services (e.g., phosgene, chlorine gas).

3. Selection Considerations and Conclusion

• Process Media: Chemical composition, toxicity, and phase.
• Operating Conditions: Temperature, pressure, and cycle frequency.
• Total Cost of Ownership: Balancing initial cost against expected service life and maintenance needs.